Q and A

HIV transmission and testing FAQ

i-Base factsheet

The i-Base Q&A resource is mainly for questions about HIV treatment. It is a service for people living with HIV. Or their partners, family or friends.

i-Base does not have the resources to answer thousands of questions about HIV risks. Or thousands of questions about HIV testing.

We do care – so we have posted everything online.

Your question will be covered by the FAQs below. Additional links are included for further information.

1. See the FAQ info below.

2. See previous questions in these categrories.

3. See this guide. Lots of info about different risks and how you can look after your health.

Do I have HIV?

The only way for you to know this is to take an HIV test. Although websites can include information about different levels of risk, no website can answer whether or not you have HIV.

This page includes information on how long you need to wait before you test. It expalins how this depends on the type of test you use.

Info on different types of HIV tests.

What is my risk for having caught HIV?

We get many questions about different risks and the likelihood of having caught HIV.

HIV is mainly transmitted sexually and by sharing drug-using equipment. Body fluids that can be infectious are blood, semen, vaginal fluid and breastmilk. However, these fluids are not infectious for very long outside the body. Within a few minutes these fluids are no longer infectious.

HIV is not transmitted by everyday contact or from contact with objects. HIV is not transmitted by saliva, sweat, spit, urine or faeces. Tears may contain HIV but this is unlikely to be a practical route of transmission.

HIV is not transmitted by deep-kissing, or from body rubbing or contact with fluid like bkood or semen in on skin. HIV is not spread by air or by insects.

You can catch HIV by having sex without a condom – if you are not taking PrEP or the person living with HIV is not on treatment (see Treatment as Prevention).

Having sex without a condom with someone who might be HIV positive is a risk for HIV. The risk each time is likely to be very low but will depend on other factors. Depending on the type of sex this might be as low as from 1 in 100 to 1 in 500.

If your partner is taking effective HIV treatment, the risk is zero.

If you are taking PrEP, this risk will also be close to zero.

The highest risk is if someone doesn’t know they are living with HIV. For example, if this was a recent infection. In early infection, viral load can be very high. This makes someone much more infectious.

Learn about how HIV is transmitted (from AVERT).

  • If you think you may have HIV, please speak to a doctor. If you have been at risk take an HIV test. These should be free in every country.
  • i-Base does not answer questions about your risk of catching HIV.
  • Testing is easy and cheap or free. If you are worried, then like millions of other people, take a test.

What about PEP?

PEP is a course of HIV drugs taken to prevent infection.

It needs to be started soon after any risk. Ideally within hour and not later than 2-3 days (72 hours maximum).

PEP stands for post exposure prophylaxis. PEP involves taking HIV medicines for a month.

After the last dose of PEP, you need to wait 4-6 weeks before you test. In the UK this is 6 weeks. If you test earlier, then you need to confirm the results 8-12 weeks later.

More info about PEP.

Will PEP work as PrEP?

PrEP is a different way to protect against HIV.

PrEP involves taken HIV meds before and after sex.

Currently, PrEP is a combination of two HIV meds in one pill. This is usually tenofovir (TD or TAF) plus emtricitabine (FTC). PrEP can be taken in two ways,

  1. Taken as a single pill every day (by cis women and trans men and women), or
  2. Taken only when needed, using 2:1:1 dosing (by cis men).

Technically, if PEP includes TD/FTC if will work as daily PrEP for everyone for the middle two weeks. For cis men, TD/FTC will work as PrEP from about day 4 to about day 26.

Long-acting formulations of PrEP are in development that use injections or oral pills.

These include cabotegravir injections, a monthly pill with islatravir, and an annual implant (with islatravir).

More info on PrEP.

UK guide to PrEP.

Will I be positive if I slept with an HIV positive person without a condom?

HIV is a difficult virus to catch sexually.

Even if your partner is HIV positive and you did not use a condom, the risk is usually less than 1 in 100 (less than 1%). This might be less than 1 in 500.

However, it only takes one exposure to catch HIV. This means that luck and other factors are involved.

So out of thousands of people who have a risk of catching HIV, some will become positive. For some people this might be from their first risky exposure.

HIV is therefore generally a low risk event, but with potentially serious outcomes. Taking an HIV testing to know your HIV status is a good idea.

Guide to HIV transmission and testing.

What is the risk if the HIV positive person is on treatment?

Effective HIV treatment prevents HIV transmission.

If someone has an undetectable viral load on treatment (less than 50 copies/mL) there is no risk of transmission. with or without condoms.

Viral load and risk of transmission.

The PARTNER 1 study reported ZERO HIV transmission after more than 58,000 times when HIV positive people with undetectable viral load had sex WITHOUT using condoms. The PARTNER 2 study reported ZERO transmission in gay couples after approximately 77,000 times that gay couple had sex WITHOUT condoms.

Q & A on the partner study.

Do I need to take an HIV test?

The only way you can know your HIV status by taking an HIV test.

In the UK, 1 in 5 people (20%) who are HIV positive do not know their HIV status. In other countries this can be even higher.

Even if your risk was low, you can only know whether you are HIV positive or HIV negative by taking a test.

HIV testing should be a routine part of looking after your sexual health. Repeating the test every 6-12 months is important in case you are at risk in the future. Some people test more often.

Different types of HIV tests.

Do I need to take a test if my partner just tested negative?

Yes, if you want to know your HIV status, you need to take your own test.

  • You can not use your partners test results to interpret your HIV status.
  • You can only know if you have HIV by taking your own test.

You could be HIV-positive and your partner has just been lucky so far. You need to know your status to protect your partner in the future.

If your partner has just tested HIV positive, you could still be HIV negative, even if you have had unprotected sex. You need your own test results.

Can I ask another person to test?


This is about your sexual health. It is your responsibility to test.

You have no right to impose your worries about your health on another person.

Only your status is important now. This involves taking responsibility for your own health.

When can I take a test?

How soon you test depends on your risk and which test you can use.

See the link above.

In 2020 UK guidelines changed. They now say that 4th generation HIV tests (antigen/antibody) will detect 99% of infections after six weeks. This is good enough to not need a second test.

Previous guidelines used to say 95% of cases will be picked up by four weeks. A negative test at four weeks needs to be checked with a second test two months later. This is to pick up the 5% of people who take longer to generate an immune response.

In high risk exposures, especially if symptoms occur, then viral load testing is sometimes used after a week. This is for needlestick injury with someone who is definitely HIV positive, This is also sometimes used after a sexual assault.

Testing graphic

What is the window period?

The window period is the time from a risk until you can take a test. Different types of test have different window periods. 

During the window period someone who is infected wirh HIV can have a HIV negative test result.

For 4th generation antigen/antibody tests a window period of four weeks is enough to detect 95% of infections. Because 5% of people take up to three months to develop antibodies a three month window is generally referred to.

However, in 2020, UK guidelines changed to recommend using this test after six weeks. This is because 99% of infections will then be detected.

What is the window period.

Which test can I use?

In the UK, tests sent to a laboratory should be 4th generation antigen/antibody test. HIV testing is free in the UK – and most other countries. UK guidelines recommend waiting six weeks to have a test. If you do this there is then no need to have a second test to check the result.

“Rapid HIV tests” where you see the result while you wait, are 3rd generation tests. These only tell you your HIV status eight weeks ago. “Rapid” refers to the results being quick, not that you can use then after a recent risk.

Viral load test are not approved or recommended for diagnosing HIV. They are sometimes used though. For example after a very high risk especially if there are symptoms of seroconversion.

It is not recommended to use viral load to test for HIV. Although some private clinics offer them for this use, they will not give you an accurate answer.

Different types of tests.

Where can I get an HIV test in the UK?

Where to get an HIV test in the UK?.

I am worried about taking an HIV test…

It is common to worry whenever you take a test.

If it turns out that you are HIV-positive, then it is better for you to know this as early as possible. You can then access appropriate monitoring and treatment. It will also help protect your sexual partners.

If it turns out that you are HIV-negative, then knowing this for certain will stop you worrying about this aspect of life.

An HIV test will focus you on your sexual health. If it is negative, use this experience to become aware of your sexual health in the futures. If you are sexually active, and take risks in the future, repeat the test every year as part of an annual check up.

How are results given?

Your test centre should clearly explain the results of your test.

If you have questions that we not explained, or that still worry you, go back to ask that centre again.

Results are generally given as:

  • Negative / non-reactive = HIV-negative = You do not have HIV (based on the window period and assuming you have not had any more recent risks)
  • Positive / reactive = HIV-positive = You have HIV infection
  • Indeterminate = the test results was unclear and will be checked with another test

What does a number on my negative HIV test result mean?

The number (ie 0.06 or 0.64 etc) is a more accurate (quantitative) way of measuring the result other than a simple, yes or no.

  • If the number is less than 1.0 the result is negative.
  • if the result is above 1.0 the result is positive
  • If the result is very close to 1.0 then the doctor may decide a confirmatory test is required.

The number means nothing else in itself. A result of 0.3 does not mean you are more likely to become positive than a number of 0.03. Increasing numbers with several test does not mean you are more likely to be positive.

The graphic below is an example of this sort of test result.

Test result nummber

Each test set their own reference ranges. It is important to see what the range is for your specific test. A reference range from one test might not be accurate for a different type/brand.

Are HIV tests accurate?

Yes. Modern HIV tests are very accurate within the time limits that they are recommended.

A 4th generation HIV test (antigen/antibody) will detect 95% of infections at 28 days after exposure. It will detect 99% of infections at 6 weeks. At 3 months it will detect more than 99.99% of infections.

False negative results (where the test shows negative but you are actually HIV positive) are very rare from any blood test. They have been reported with some oral (saliva) tests if they are not given correctly. If you are worried about this, then a second will confirm whether the result is a test error.

A positive test result always needs to be routinely confirmed using a different type of test. When used as a confirmatory test this is close to 100% accurate.

If you are still worried, for example because of symptoms, then a viral load test will show if HIV is present. If the viral load test is negative, then the symptoms are not related to HIV.

Information about different types of tests

What is the difference between 3rd and 4th generation tests?

  • 3rd generation tests look for antibodies to HIV. Rapid tests are 3rd generation. Antibodies take time to be produced. This means that 3rd generation tests tell you about your HIV status 8-12 weeks before you took the test.
  • 4th generation tests look for both antibodies and antigens (ie direct evidence of HIV). HIV is detectable before antibodies are produced. This means that 4th generation tests tell you about your HIV status 3-6 weeks before you took the test. It is only at 6 weeks though that 99% of cases will be picked up

A few studies have suggested that 3rd generation testing might be accurate earlier than the explanation above.

What about rapid HIV tests?

Rapid HIV tests are 3rd generation tests. They only give an accurate result 8 weeks on longer after any risk.

What if my first test is positive and the second test is negative?

Please talk to your test centre doctor.

i-Base cannot interpret individual test results. This is something that your doctor or test centre has to do. The interpetation will depend on the type of test, your risk, and the time between the risk and testing.

Usually the second test will be the accurate result – but this is something to talk the doctor or clinic about.

How accurate is a test at 14, 18, 24, 27, 49, 100 days etc?

Test accuracy data based on 28 days. It is not available at other time points. Information on the i-Base website is based on 4th generation tests being 95% accurate at 4 weeks, and that this needs to be confirmed after three months with a second test. A negative text after three months (90 days) mean you are HIV negative.

  • Testing earlier is less accurate, but can help if someone is anxious.
  • Later testing later is more accurate, but involves a longer wait.

Test accuracy at different time points is difficult to estimate and study. It requires each type of HIV test being checked on blood samples from several hundred people who were donating blood frequently (usually twice a week) and who then became infected without realising it.

Only a small proportion of people regularly give blood and only a small proportion of those people catch HIV. These sample are difficult to get. Each manufacturer uses a similar bank of blood samples to estimate the sensitivity of their test.

Some samples show a positive result after only a week, others take several months. This is because of the wide range of individual responses to an infection.

Does a negative result mean I do not have HIV? Is the result conclusive?

Yes if the test was three months after your exposure risk. This is the purpose to testing. If the result is negative, and you have not had another risk, you do not have HIV. Stop worrying. Learn from the experience you have gone through taking a test. Learn about how to protect yourself. Look after your sexual health in the future.

If your test was less than three months after the exposure then you might need to test again after three months. This depends on your risk.

If you tested negative four weeks after the exposure, you a very likely to be HIV-negative. In the UK, whether or not you need to confirm the result with a second test several months later depends on your risk. This is because 5% of people can take longer than four weeks to generate an immune response.

Is a negative result 100% accurate?

We get asked this a lot. As an information service we have to say that technically, no test is 100% accurate.

This is partly to do with language. In science you can never prove 100% that something is NOT there or that something will NEVER happen. This is referred to as “the impossibility of proving a negative”.

In practice, a negative result three months after your last exposure risk is interpreted to mean you do NOT have HIV.

In real life, this means you do NOT have HIV.

If you have symptoms, this is not likely to be due to HIV. If they were related to HIV you would test positive. The symptoms are related to something else – see a doctor.

What is a ‘false negative’ test result?

A false negative test result occurs when the test shows negative and the person is really HIV positive.

This is very rare. It usually occurs during the window period when someone is very recently infected but the test can’t pick up the infection.

Nearly all tests have a margin of error. this is usually small. With HIV antibody tests only 0.3% of tests (3 tests in every 1000) will be a false negative after 3 months.

With fourth generation tests, it is lower still at 3 months.

If you are worried you can take a second test. This will eliminate the possibility of a false negative result.

What is a ‘false positive’ test result?

A false positive test is when the test result shows positive but the person is negative.

This can happen with antibody tests when the test picks up antibodies for other infections.

Approximately 15 out of every 1000 antibody tests are a false positive (1.5%).

The fourth generation tests have a much lower chance of a false positive. This is why you should always have a second confirmatory test if the result is positive.

The test used to confirm a positive result is 100% accurate. If the second test does not confirm you are positive then you do not have HIV.

Do I need to take another test?

No, not if your exposure risk was more than three months prior to the test.

If the test was taken less than three months after the exposure risk, then retesting three months after the exposure risk is recommended in most guidelines.

Why do I need another test at 3 months after an exposure?

Please follow this link for a very comprehensive answer regarding the BASHH guidelines and why it is that some GUM clinics still ask patients to do a confirmatory test at 3 months.

Why do some doctors (and websites) say to test again at 6 months?

We are based in the UK and our information is based on UK guidelines. There are occasional reports where people may take longer than three months to develop antibodies to HIV. If they occur these are very rare.

A negative test at three months means it is fine to get on with your life. The experience of taking an HIV test should help focus you on taking fewer risks in the future. It should also make it easier to routinely include an HIV test in your future sexual health care.

Whether this is after six months or every year will depend on how often you have sex that includes the risk of HIV.

Testing is usually free and easy. You will not do any harm if you want to test at six months. Just don’t spend the time obsessing over HIV because a negative test at three months is very good news!

Can meds or other illnesses or things affect the result of my HIV test?

HIV antibody tests are not affected by other infections, medications, vaccinations, putting on weight, eating or drinking anything before the test, use of alcohol or recreational drugs, mouthwash or time of day.

Your test result is accurate even if you had flu or a cold or are using antihistamine treatment, for example, for hay fever.

You do not need to fast before your test, eating and drinking before the test will not affect the results.

How can my partner test positive and I test negative?

Why different people test positive and negative.

What is seroconversion? What are the symptoms?

Information on seroconversion.

Does this symptom [… rash/itch/headache etc] related to HIV?

Most people worried about a recent infection mistake common symptoms of other illnesses with HIV. Stress and worry can cause and contribute to these symptoms. Information about symptoms of recent HIV infection.

If you have any symptoms that are worrying you, then seek medical advice from a doctor or other health care worker. The people working at i-Base are not doctors. We cannot diagnose HIV or any other illness.

Where can I get more information about risks of catching HIV?

This online guide has a lot more infromation about different risks.

Ways you can NOT catch HIV

The following list includes examples or questions we get from people worried about catching HIV.

The information above should have clear enough these are not risks. Similar questions will not be answered in the Q&A pages but may just be added to this list below.

You can NOT catch HIV from:

  • Masturbation.
  • Sexual activity with someone who does not have HIV.
  • Eating prepacked chicken which may have had human blood on it.
  • Eating any food cooked or uncooked with blood on it.
  • From a sterile needle at a GUM clinic or any other healthcare centre.
  • From a human bite.
  • From an insect bite including a mosquito bite.
  • From an animal.
  • From being in close proximity to a ‘sharps’ bin in a GUM clinic.
  • From living in the same house as someone who is HIV positive (unless you have unprotected sex).
  • From a sewing needle if you stab your finger.
  • Blood on a bus seat that went through underwear.
  • Cleaning nail clippers.
  • Using a knife/fork/spoon/cup/plate etc.
  • From a plaster.
  • From a razor that hasn’t been used for hours (probably even minutes).
  • From eating peanuts from a bowl.

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Last updated: 4 April 2023.